1. Field of the Invention
This invention relates generally to the transfer of fluids, electric and hydraulic power, and signals between installations on the sea bed and a weathervaning vessel that is moored to a single point mooring. The primary application of the invention is for offshore oil production vessels that are moored to a single point mooring or a turret and to oil tankers that are moored to single point moorings.
The invention relates to the fluid swivel required in the conduits and cables connecting the systems aboard the weathervaning vessel to the sub sea systems.
2. BACKGROUND ART
In recent years a number of systems have been developed to moor vessels offshore in high sea states. These systems generally provide for the mooring of oil tankers and oil production and storage vessels such that they are moored to a single point (such as a single point mooring or a turret mooring). The vessels are typically designed to be able to freely weather vane about the mooring. Typically the vessel is connected with fluid, power, and signal connections to sub sea systems. Because the vessel can freely rotate, swivels must be incorporated in the said fluid, power, and signal connections.
Typically there are 5 to 10 fluid connections and a large number of signal connections, however, the number of fluid connections may range from one to more than 30. A number of solutions to this swivel problem exists in the known art as follows:
1. Multiple ball bearing or roller bearing type swivels stacked one on top of each other such that they all have one common (usually vertical) axis. All conduits must pass through the center of the lowest swivel, all but one must pass through the center of the next lowest swivel, and so on.
2. Numerous flexible conduits of the type manufactured by COFLEXIP deployed in a cylindrical drum and subject to twisting, such as for example described in U.S. Pat. Nos. 5,201,551 and 4,648,848.
3. Numerous flexible conduits of the type manufactured by COFLEXIP reeled onto or off a drum with a vertical axis and that are maintained in tension by an idler drum with a vertical axis and moved horizontally with respect to the first drum as described in U.S. Pat. No. 4,436,048.
4. Numerous flexible conduits of the type manufactured by COFLEXIP deployed spirally on a plane or slightly conical surface where the two ends of the conduits rotate relative to one another by winding or unwinding the spiral similar to a clock spring as described in U.S. Pat. No. 4,015,416.
5. Numerous flexible conduits of the type manufactured by COFLEXIP transferred between two coaxially deployed cylinders and being transferred between the two cylinders by means of an intermediate spooling device mechanically coupled to both cylinders such that the spooling device rotates with the average rotational speed of the two cylinders as described in U.S. Pat. Nos. 4,643,462 and 4,597,595.
Swivels of the first type can rotate an unlimited number of revolutions in either direction. However, for multiple paths this type of swivel has the disadvantage of being heavy and prone to leaks. It also has limited internal pressure capability of up to about 35 MPa.
Swivels of the types 2 through 5 are all limited number of revolutions swivels. They can practically only be designed for 6 to 8 revolutions before reaching the limit. They are all heavy with the exception of type 5. Type 5 as described in U.S. Pat. Nos. 4,643,462 and 4,597,595 has a practical limit of revolutions on the order of 4 to 6 revolutions.
An object of the present invention is to make a limited number of revolutions swivel permitting a large number of revolutions such as 20 and permitting a large number of fluid, power, and signal paths. A further object is to make the swivel leak proof and capable of resisting large internal pressures in the conveyed fluid such as 70 MPa.
These and other objects are achieved by providing a swivel of type 5 as described above but rather than spooling the pipes between two cylinders the pipes are spooled between two cones in such a manner that the pipe on one cone is spooled from the top down and on the other cone from the bottom up. This method of spooling ensures a constant distance between the unwinding point on one cone and the winding point on the other cone. By providing cones with the apex pointing up the spooled flexible pipe on either cone is prevented from falling down.
An alternative embodiment spools between two cylinders with a common axis. In this embodiment the pipe spooled on one cylinder is supported by a structure resting on the pipe spooled on the other cylinder, thereby preventing the pipe on either cylinder from falling down.
Yet another embodiment spools between two cylinders provided with helically formed shelves. In this embodiment the pipes spooled on the cylinders are supported by the helically formed shelves.
The invention removes the limitation on existing type 5 swivels of only being able to practically store two wraps of pipe on each spool to permitting 10 or more wraps on each spool thereby making the number of revolutions end to end 20 or more. Because there are no rotating seals in the swivel according to this invention and because the swivel may use pipe of the type manufactured by COFLEXIP, the pressure limitations for the swivel are basically the same as for COFLEXIP type pipe.